SQL using select with group by

SELECT manager_id, COUNT(manager_id)
FROM employees
GROUP BY manager_id
HAVING COUNT(manager_id) > 3

I have no problem with the code, but,I want to display the manager first name and last name instead of the manager_id, however if I do this, I get the error:
"column 'employees.first_name' is invalid... not contained in either an aggregate function or group by clause". I tried adding first_name and last_name to the group by, output being blank. Tried self join as well.. can't figure out the answer.
The query includes: employee_id, first_name, last_name, manager_id